With recent advances in various technologies in the field of silver halide photosensitive materials, photosensitive materials have come to the market in recent years which have sensitivities exceeding 400 or even 1000 on the ISO scale. There is a demand, however, for even higher sensitivities for photosensitive materials which are used in photography in dark rooms without a flash, in high shutter speed photography using telephoto lenses such as in sports photography, and in long exposure photography such as in astronomical photography. Thus, the perpetual goal facing this industry is to continue to expand the range of photography through the development of even higher sensitivity photographic materials.
A great deal of effort has been put into making photosensitive materials of even higher sensitivity. Some of these methods involve changing the shape of the silver halide particles, increasing chemical sensitivity, increasing the spectral sensitivity, using additives, or altering the coupler structure. Several useful inventions have resulted from this research. However, the requirements for high sensitivity photosensitive materials has outstripped the pace of the progress. The regrettable truth is that the above methods remain insufficient to fulfill these requirements. In this industry, the usual method for preparing high sensitivity photosensitive materials is to increase the size of the silver halide emulsion particles in conjunction with using some additional technology. While sensitivity can be increased to a certain degree by increasing the particle size in the silver halide emulsion, so long as the amount of the silver halide is kept constant, the natural result from this is to reduce the number of the silver halide emulsion particles, thereby reducing the developing initiator points, and deteriorating graininess. Methods which have been proposed to improve upon this situation include British Patent 923,045 and JP-B-49-15495 (the term "JP-B" as used herein means an "examined Japanese patent publication") which propose photographic materials having two or more emulsion layers having the same color sensitivity but having differing sensitivity i.e., having differing silver halide particle sizes in the emulsions; JP-A-55-62454 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") which proposes using a coupler having high speed reactivity; U.S. Pat. No. 3,227,554 and U.S. Pat. No. 3,632,435 which propose using a DIR compound and a DIR coupler; British Patent No. 2,083,640 which proposes using a coupler which is able to produce a mobile dye; and JP-A-60-128443 which proposes using a silver halide having a high average silver iodide content. These methods all produce good results in their respective areas and excellent inventions, but the technology they offered is still insufficient to satisfy the demands for obtaining higher sensitivity and higher image quality. At this point, in order to increase the number of developing initiator points with increasing the size of the silver halide emulsion particles, the content of silver halite particles of a high sensitivity color negative photographic materials have been increased in such an amount that characteristics such as desilvering property during a bleach fixing process are not deteriorated.
However, when the above process was used to obtain high sensitivity, high image quality photosensitive materials, it was found that there were a number of undesirable properties associated with it. For example, it was found that in the period during the time following the manufacture until the materials were used, fogging increases and graininess also increases to degrade the photographic properties of the materials. In particular, the increase in fogging was so dramatic as to pose problems in using the materials. It has been reported that the reasons for the increase in fogging of photosensitive materials during long periods of time, in addition to the normal reasons of heat and humidity induced fogging, involves what is called environmental radiation from .gamma.-rays and irradiation from space. However, the inventors own research has recently indicated that there may be additional factors involved in this increase in fogging. As a result of exhaustive research on this matter, the inventors have discovered that one other reason for the fogging is the quantity of potassium ion in the photosensitive materials.
This potassium ion is introduced into the silver halide emulsion when it is prepared or when the pAg of the emulsion is controlled, as KCl, KBr, KI, or as a part of dyes, gelatin or reagents which are added to the photosensitive materials. It was surprising indeed to find that when large quantities of potassium were present in the photosensitive materials their properties would decline with the passage of time. Since there had been no previous finding of the adverse effects caused by potassium ion in the photosensitive materials, any countermeasure had not been established with respect to the amount of potassium, and more particularly the amount of potassium ion, in photosensitive materials and that fact has been a big problem.